Most PSM are stored in plants in relatively high concentrations, which can exceed
more than 10% of total dry weight. Water-soluble PSM are usually stored in
the vacuole of plant cells, often the epidermal cells. Lipophilic compounds are
not sequestered in the vacuole but excreted into raisin ducts, lacticifers, oil cells,
trichomes, dead cells, or the cuticle [5, 11, 12].
PSM can be synthesized in almost any plant organ, but this differs from plant
species to another or types of PSM. Some plants produce a PSM in the root, but store
it in the aerial parts (e.g., tropane alkaloids or Nicotiana alkaloids). In order to
reach the aerial parts, PSM undergo long-distance transport via the xylem. Other
plants produce PSM in the leaves, sometimes even in chloroplasts. If these PSM are
accumulated in roots, stems, flowers, or seeds, they are usually transported there via
the phloem (e.g., quinolizidine alkaloids) [11].
Plants usually do not produce a single PSM but a couple of main PSM and several
derivatives, differing by additional hydroxyl, methoxy, epoxy, aldehyde, or ester
moieties or the degree of oxidation. These additional functional groups can influence
the biological activity of a PSM [13, 14]. In many instances, plants not only
produce a mixture of PSM from the same class of PSM but from several classes.
The composition of these mixtures differs between plant organs, i.e., PSM of roots
differ from those of leaves or seeds. Furthermore, PSM profiles differ between
developmental stages; i.e., profiles from seedlings differ from those of mature
flowering or senescing plants [11]. This feature is important for our discussion of
co-evolution of angiosperms and herbivores. Within a population, a substantial
variation in content and composition of PSM can be expected. This variation can
be due to genetic or environmental factors. Plants growing at a sunny site have a
different PSM profile from those living in the shade, or mountain populations
differ from low-land populations.
Most PSM are not end products but can undergo metabolism. Nitrogen-containing PSM are often used as nitrogen-storage compounds by some plants.
Legumes (family Fabaceae), which store quinolizidine alkaloids, lectins, protease
inhibitors, alkaloids, or nonprotein amino acids in their seeds, mobilize these PSM
after germination and use their nitrogen for the developing young plant (Fig. 2). This
feature is important, because nitrogen is a limiting factor for most plants and thus its
use must be economic [2, 5, 11].
Plant secondary metabolism is not static. When plants are infected by a
pathogen or wounded by an herbivore (an animals which feeds on plants), the
secondary metabolism is often activated. Preformed PSMs are activated after
cleavage of a sugar moiety by beta glucosidase or an esterase; examples are
cyanogenic glucosides, glucosinolates, saponins, or flavonoid glycosides
[15–17]. In other cases, the formation of existing PSMs is stimulated or in some
plants completely new PSM are synthesized, mostly oriented against microorganisms. Plant pathologist have termed the new compounds “phytoalexins.” Plant
hormones, such as gibberellic acid, jasmonic acid, salicylic acid, play important
roles in the corresponding signal pathways (involving calcium signaling) and
differential gene expression, which regulate plant responses to the environment
[11, 15, 18].
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
153
more than 10% of total dry weight. Water-soluble PSM are usually stored in
the vacuole of plant cells, often the epidermal cells. Lipophilic compounds are
not sequestered in the vacuole but excreted into raisin ducts, lacticifers, oil cells,
trichomes, dead cells, or the cuticle [5, 11, 12].
PSM can be synthesized in almost any plant organ, but this differs from plant
species to another or types of PSM. Some plants produce a PSM in the root, but store
it in the aerial parts (e.g., tropane alkaloids or Nicotiana alkaloids). In order to
reach the aerial parts, PSM undergo long-distance transport via the xylem. Other
plants produce PSM in the leaves, sometimes even in chloroplasts. If these PSM are
accumulated in roots, stems, flowers, or seeds, they are usually transported there via
the phloem (e.g., quinolizidine alkaloids) [11].
Plants usually do not produce a single PSM but a couple of main PSM and several
derivatives, differing by additional hydroxyl, methoxy, epoxy, aldehyde, or ester
moieties or the degree of oxidation. These additional functional groups can influence
the biological activity of a PSM [13, 14]. In many instances, plants not only
produce a mixture of PSM from the same class of PSM but from several classes.
The composition of these mixtures differs between plant organs, i.e., PSM of roots
differ from those of leaves or seeds. Furthermore, PSM profiles differ between
developmental stages; i.e., profiles from seedlings differ from those of mature
flowering or senescing plants [11]. This feature is important for our discussion of
co-evolution of angiosperms and herbivores. Within a population, a substantial
variation in content and composition of PSM can be expected. This variation can
be due to genetic or environmental factors. Plants growing at a sunny site have a
different PSM profile from those living in the shade, or mountain populations
differ from low-land populations.
Most PSM are not end products but can undergo metabolism. Nitrogen-containing PSM are often used as nitrogen-storage compounds by some plants.
Legumes (family Fabaceae), which store quinolizidine alkaloids, lectins, protease
inhibitors, alkaloids, or nonprotein amino acids in their seeds, mobilize these PSM
after germination and use their nitrogen for the developing young plant (Fig. 2). This
feature is important, because nitrogen is a limiting factor for most plants and thus its
use must be economic [2, 5, 11].
Plant secondary metabolism is not static. When plants are infected by a
pathogen or wounded by an herbivore (an animals which feeds on plants), the
secondary metabolism is often activated. Preformed PSMs are activated after
cleavage of a sugar moiety by beta glucosidase or an esterase; examples are
cyanogenic glucosides, glucosinolates, saponins, or flavonoid glycosides
[15–17]. In other cases, the formation of existing PSMs is stimulated or in some
plants completely new PSM are synthesized, mostly oriented against microorganisms. Plant pathologist have termed the new compounds “phytoalexins.” Plant
hormones, such as gibberellic acid, jasmonic acid, salicylic acid, play important
roles in the corresponding signal pathways (involving calcium signaling) and
differential gene expression, which regulate plant responses to the environment
[11, 15, 18].
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
153